Continuous-flow high-pressure waterjets are well known in the art for cleaning and cutting applications. Depending on the particular application, the water pressure required to produce a high-pressure waterjet may be in the order of a few thousand pounds per square inch (psi) for fairly straightforward cleaning tasks to tens of thousands of pounds per square inch for cutting and removing hardened coatings.
Examples of continuous-flow, high-pressure waterjet systems for cutting and cleaning are disclosed in U.S. Pat. No. 4,787,178 (Morgan et al.), U.S. Pat. No. 4,966,059 (Landeck), U.S. Pat. No. 6,533,640 (Nopwaskey et al.), U.S. Pat. No. 5,584,016 (Varghese et al.), U.S. Pat. No. 5,778,713 (Butler et al.), U.S. Pat. No. 6,021,699 (Caspar), U.S. Pat. No. 6,126,524 (Shepherd) and U.S. Pat. No. 6,220,529 (Xu). Further examples are found in European Patent Applications EP 0 810 038 (Munoz) and EP 0 983 827 (Zumstein), as well as in U.S. Patent Application Publications US 2002/0109017 (Rogers et al.), US 2002/0124868 (Rice et al.), and US 2002/0173220 (Lewin et al.).
Continuous-flow waterjet technology, of which the foregoing are examples, suffers from certain drawbacks which render continuous-flow waterjet systems expensive and cumbersome. As persons skilled in the art have come to appreciate, continuous-flow waterjet equipment must be robustly designed to withstand the extremely high water pressures involved. Consequently, the nozzle, water lines and fittings are bulky, heavy and expensive. To deliver an ultra-high-pressure waterjet, an expensive ultra-high-pressure water pump is required, which further increases costs both in terms of the capital cost of such a pump and the energy costs associated with running such a pump.
In response to the shortcomings of continuous-flow waterjets, an ultrasonically pulsating nozzle was developed to deliver high-frequency modulated water in non-continuous, virtually discrete packets, or “slugs”. This ultrasonic nozzle is described and illustrated in detail in U.S. Pat. No. 5,154,347 (Vijay) which issued on Oct. 13, 1992. The ultrasonic nozzle disclosed in U.S. Pat. No. 5,154,347 transduced ultrasonic oscillations from an ultrasonic generator into ultra-high frequency mechanical vibrations capable of imparting thousands of pulses per second to the waterjet as it travels through the nozzle. The waterjet pulses impart a waterhammer pressure onto the surface to be cut or cleaned. Because of this rapid bombardment of mini-slugs of water, each imparting a waterhammer pressure on the target surface, the erosive capacity of the waterjet is tremendously enhanced. The ultrasonically pulsating nozzle which cuts or cleans is thus able to cut or clean much more efficiently than the prior-art continuous-flow waterjets.
Theoretically, the erosive pressure striking the target surface is the stagnation pressure, or 1/2.rho.v.sup.2 (where ρ represents the water density and v represents the impact velocity of the water as it impinges on the target surface). The pressure arising due to the waterhammer phenomenon, by contrast, is ρcv (where c represents the speed of sound in water, which is approximately 1524 m/s). Thus, the theoretical magnification of impact pressure achieved by pulsating the waterjet is 2 c/v. Even if air drag neglected and the impact velocity is assumed to approximate the fluid discharge velocity of 1500 feet per second (or approximately 465 m/s), the magnification of impact pressure is about 6 to 7. If the model takes into account air drag and the impact velocity is about 300 m/s, then the theoretical magnification would be tenfold.
In practice, due to frictional losses and other inefficiencies, the pulsating ultrasonic nozzle described in U.S. Pat. No. 5,154,347 imparts about 6 to 8 times more impact pressure onto the target surface for a given source pressure. Therefore, to achieve the same erosive capacity, the pulsating nozzle need only operate with a pressure source that is 6 to 8 times less powerful. Since the pulsating nozzle may be used with a much smaller and less expensive pump, it is more economical than continuous-flow waterjet nozzles. Further, since waterjet pressure in the nozzle, lines, and fittings is much less with an ultrasonic nozzle, the ultrasonic nozzle can be designed to be lighter, less cumbersome and more cost-effective.
Although the ultrasonic nozzle described in U.S. Pat. No. 5,154,347 represented a substantial breakthrough in waterjet cutting and cleaning technology, further refinements and improvements were found by the Applicant to be desirable. The first iteration of the ultrasonic nozzle, which is described in U.S. Pat. No. 5,154,347, proved to be sub-optimal because it was used in conjunction with pre-existing waterjet generators. A need therefore arose for a complete ultrasonic waterjet apparatus which takes full advantage of the ultrasonic nozzle.
It also proved desirable to modify the ultrasonic nozzle to make it more efficient from a fluid-dynamic perspective, to be able to clean and remove coatings more efficiently from large surfaces, and to be more ergonomic in the hands of the end-user.
Accordingly, in light of the foregoing deficiencies, it would be highly desirable to provide an improved ultrasonic waterjet apparatus.